1. Field of the Invention
The present invention relates to a blind rivet, and more particularly to an improvement in the so-called `peel` type blind rivet specially adapted for use to fasten a relatively soft article to a hard support or the like.
2. Prior Art
The blind rivet of the peel type is composed of a rivet body and a mandrel with a head, wherein radial blades are formed in and integral with the lower surface of this head. The rivet body comprises a cylindrical part that has at one of its opposite ends a flange-shaped head. In use, the mandrel inserted in the cylindrical part of the rivet body will be pulled strongly to such an extent that the blades cut deep into the other open end of said cylindrical part. A petal-shaped flare, that will thus be formed at that other end, consists of a plurality of severed feet protruding centrifugally so that these feet cooperate with the flange-shaped head to fasten a soft article onto a hard support (see for example the Japanese Laid-Open Patent No. 2-26305, ibid. 10-274220 and the Laid-Open Utility Model No. 5-36115).
In each of the prior art blind rivets of the described type, the mandrel head is of a diameter substantially equal to or slightly smaller than outer diameter of the cylindrical part of the rivet body. Therefore, effective cutting area defined as the diameter of a circumcircle surrounding the blades in the lower surface of mandrel head is noticeably smaller than said outer diameter of the cylindrical part. As a result, those blades are likely to fail to cut full thickness of peripheral wall at the open end of said cylindrical part. Thus, it has been difficult to cut the end wall into severed feet, and even if this were possible, those feet have tended to form a distorted or irregular flare at said end.
In the blind rivet disclosed in the Laid-Open Utility Model No. 5-36115, grooves are formed in the inner periphery of the rivet body's cylindrical part, at the open end thereof These grooves extending longitudinally of said part are weakened zones where the peripheral wall is rendered easy to break. However, such grooves are not only difficult to form, but also make it necessary for the mandrel's blades to be aligned with the respective grooves in the rivet body. Misalignment of the blades with the grooves will give rise to another problem that the cylindrical part's open end becomes more difficult to cut.
In the other prior art blind rivet shown in the Laid-Open Patent No. 2-26305, the peripheral wall of the open end is thinned for easier cutting with the blades. A petal-shaped flare, that may be formed in use in such a weakened region, will however probably fail to provide a sufficient fastening strength.
It is almost impossible to apply those prior art structures to blind rivets whose rivet bodies are made of an ordinary steel or a stainless steel, even if feasible for blind rivets of a much softer material such as aluminum alloys easy to cut.